Forms plugin validate.message

I'm not sure if I'm misunderstanding what this is meant to do, I'm guessing it's where you enter your validation messages but I can't get it to work. I'm just getting the HTML5 validation messages.

Any ideas?
Thanks

The message will show when there is only server-side validation. Most fields validation also happens in the client-side (browser).

Thanks, I've got these working now. Not entirely sure how, I installed the jQuery validation plugin and they started showing. I'll play with it some more and figure it out.